After running the report of Top Senders, many senders are shown as (none)
The NDR message does not have a sender. Therefore, the sender value is set to "none."
The report is working as designed.
You may track null sender messages using the Message Audit Log by doing the following if you desire more details:
This will allow you to find all messages that were not included in the report due to a null verdict such as Virus/Spam.